Heretofore, electrolytic electrodes comprising a substrate of valve metals, e.g., titanium (Ti), have been used as superior insoluble metal electrodes in the field of electrochemistry. In particular, they have been widely used as anodes for the generation of chlorine in the salt (sodium chloride) electrolytic industry. In addition to Ti, tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), vanadium (V), molybdenum (Mo), tungsten (W), etc. are known as valve metals.
These metal electrodes are produced by coating metallic titanium with various electrochemically active substances such as platinum group metals and their oxides. Examples of such platinum group metals and their oxides are described in, e.g., U.S. Pat. Nos. 3,632,498 and 3,711,385. These electrodes can maintain a low chlorine overvoltage over a long period of time as electrodes for the generation of chlorine.
However, when the above metal electrodes are used as anodes in electrolysis for the generation of oxygen or electrolysis in which the generation of oxygen is involved, the anode overvoltage gradually increases. In extreme cases, the anode is passivated and thus it becomes impossible to continue the electrolysis.
The phenomenon of passivation of the anode is believed to be caused mainly by the formation of electrically non-conductive titanium oxides that result from (1) the oxidation of the titanium base material with oxygen by the electrode coating-constituting oxide substance itself; (2) oxygen diffusion-permeating through the electrode coating; or (3) the electrolyte.
Formation of such electrically non-conductive oxides in the interface between the base material and the electrode coating causes the electrode coating to peel off. This creates problems such as a breakdown of the electrode.
Electrochemical processes in which the anode product is oxygen, or where oxygen is generated at the anode as a side reaction, include: (1) electrolysis using a sulfuric acid bath, a nitric acid bath, an alkali bath or the like; (2) electrolytic separation of chromium (Cr), copper (Cu), zinc (Zn), or the like; (3) various types of electroplating; (4) electrolysis of dilute salt water, sea water, hydrochloric acid, or the like; and (5) electrolysis for the production of chlorate, and so forth. These processes are all industrially important. However, the above-described problems have hindered metal electrodes from being used in these processes.
U.S. Pat. No. 3,775,284 discloses a technique to overcome passivation of the electrode due to permeation of oxygen. In this technique, a barrier layer of a platinum (Pt)-iridium (Ir) alloy, or of an oxide of cobalt (Co), manganese (Mn), lead (Pb), palladium (Pd), and Pt is provided between the electrically-conductive substrate and the electrode coating.
The substances forming the intermediate barrier layer prevent the diffusion-permeation of oxygen during electrolysis to some extent. However, these substances are electrochemically very active and therefore, react with the electrolyte passing through the electrode coating. This produces electrolytic products, e.g., gas, on the surface of the intermediate barrier layer which gives rise to additional problems. For example, the adhesion of the electrode coating is deteriorated due to physical and chemical influences of the electrode coating peeling off before the life of the substance of the electrode coating is over. Another problem is the corrosion resistance of the resulting electrodes is poor. Thus, the method disclosed in U.S. Pat. No. 3,775,284 fails to produce electrolytic electrodes which have high durability.
Japanese Patent Appliction (OPI) No. 40381/76 (the term "OPI" used herein refers to a "Published Unexamined Patent Application") discloses an intermediate coating layer comprising tin oxide doped with antimony oxide for coating the anode. However, the anode used is an anode intended for the generation of chlorine, and hence an electrode provided with an intermediate coating forming substance disclosed in the above publication does not show the generation of oxygen.
U.S. Pat. No. 3,773,555 discloses an electrode in which a layer of an oxide of, e.g., Ti, and a layer of a platinum group metal or an oxide thereof are laminated and coated on the electrode. However, this electrode has the problem that when it is used in electrolysis in which the generation of oxygen is involved, passivation occurs.